1. Field of the Invention
This invention relates in general to the field of pressurized light water nuclear reactors and in particular to the radial neutron reflector surrounding the nuclear core for improved neutron economy to lower fuel costs.
2. Description of the Prior Art
It is well known that commercial pressurized light water nuclear reactors are both a technical and commercial success. In such reactors, a reactive region commonly referred to as a nuclear core contains fissile fuel such as uranium 235 or plutonium 239 in which sustained fission reactions occur to generate heat. A group of mechanical components which are known as reactor internals structurally support the core within a hermetically sealed pressure vessel. The reactor internals also direct the flow of a cooling medium such as light water through the nuclear core, and out of the pressure vessel. The cooling medium which is alternatively called the reactor coolant removes the heat generated by the nuclear core and transfers the heat to another cooling medium within heat exchangers which are located external of the pressure vessel. The second cooling medium is usually water which is converted into steam in the heat exchangers and is used to produce electricity by conventional steam turbine-electrical generator combinations.
In general, in such reactors, the nuclear core is comprised of a plurality of elongated fuel assemblies having a substantially square cross section. The reactor internals usually comprise an upper core support plate, a lower core support plate and a core barrel. The upper core support plate is supported by a flange within the pressure vessel. The lower core support plate is attached to the core barrel, at its lower end. The upper portion of the core barrel is also supported by a flange within the pressure vessel. The core barrel comprises an elongated cylinder interposed between the nuclear core and the cylindrical wall of the pressure vessel. The nuclear core is positioned within the core barrel and between the upper and lower core support plates. Typically, the reactor coolant enters the pressure vessel through one or more inlet nozzles, flows downward between the pressure vessel and the outside of the core barrel, turns 180.degree., and flows upward through the lower core support plate and through the core. The heated reactor coolant then turns 90.degree. and exits the pressure vessel through one or more exit nozzles and then to the heat exchangers previously mentioned.
In any reactor, such as the one described, the fission rate of the nuclear fuel or the number of neutrons produced by the fission process must remain constant. It is well known that each neutron producing a fission causes heat and the production of more than one other neutron. To sustain the nuclear chain reaction, at least one of the newly produced neutrons must then fission another atom of fuel. The reactor coolant comprising light water is an excellent moderator of neutrons; hence, it is the primary means by which the fast neutrons produced by the fission process are thermalized or slowed down so that another fission may occur and thereby sustain the chain reaction. The excess neutrons are accounted for in a number of different ways. Some are slowed down and absorbed by a nuclear poison such as boron which is dissolved in the primary coolant. Others are absorbed by axially movable control rods which are interpersed throughout the nuclear core and made of materials specifically selected to absorb neutrons. Control rods are well known in the prior art and comprise the primary means to control the operating power level of the nuclear reactor. Still other neutrons are absorbed by poisons which buildup within the nuclear fuel and are caused by the fission process itself.
In order to extend the life of the nuclear core as long as is practical so as to minimize time consuming reactor shutdowns for refueling purposes, the fuel assemblies are provided with enriched nuclear fuel, usually enriched uranium 235. This excessive amount of reactivity is designed into the core at startup so that as the reactivity is depleted over the life of the core, the excess reactivity may then be used to extend the life of the core. The amount of enrichment continuously decreases as the reactor operates until such time as the fuel can no longer sustain the chain reaction. Then the reactor must be shut down and refueled. During the initial stages of reactor operation or during the phase which is known as beginning of life, special neutron absorbing control rods are inserted within the core and/or additional soluble poisons are dissolved within the reactor coolant to absorb the excess reactivity. As the excess reactivity decreases, the amount of insertion of the special control rods and/or the amount of soluble poison is decreased to allow use of the excess reactivity. In this manner, the excess reactivity is held in abeyance until it is needed.
Enriched uranium is extremely expensive. It is preferable therefore to reduce the amount of enrichment whenever possible but without reducing the extended length of the life of the core. One recognized method of theory to accomplish this result is by making more efficient use of the neutrons produced by the fission process. An area where present day nuclear reactors are relatively inefficient as regards neutron economy is concerned is the region of the reactor between the internal diameter of the pressure vessel and the core and in particular between the internal diameter of the core barrel and the outer periphery of the fuel assemblies. Since the fuel assemblies are square in cross section, side-by-side stacking of the fuel assemblies produces an irregular noncircular outer periphery of the core. Typically, stainless steel vertical plates are positioned against the irregular periphery of the core. The vertical plates are supported by a plurality of horizontal "former plates" bolted to the vertical reflector plates. The former plates are in turn bolted to the core barrel.
The former plates are specially shaped to provide for the transition from the irregular core periphery to the circular shape of the core barrel. The vertical plates provide for core lateral support and prevent the reactor coolant from bypassing the core. Although not originally intended, it has been determined that the vertical stainless steel plates also provide a radial neutron reflection function. In this manner, means have been provided whereby some of the excess neutrons produced by the fission process and which radially escape from the core are reflected by the stainless steel plates back into the core. Unfortunately, the present day design of the stainless steel plates as regards the radial reflector function is not as efficient as desired. For example, the space between the vertical stainless steel and the horizontal former plates is occupied by primary coolant which allows for removal of the heat absorbed by the core barrel, the former plates and the vertical plates. While water is an excellent moderator it is an inefficient reflector. Thus, while some neutrons are reflected back into the core, a great number are thermalized and/or absorbed by the relatively large volume of reactor coolant located radially external of the nuclear core.
Accordingly, a primary object of the present invention is to provide apparatus surrounding the irregular shape of the nuclear core which provides for core lateral support, prevents the reactor coolant from bypassing the core and which reduces the net amount of neutron leakage from the core by reflecting neutrons which would otherwise escape, back into the core for improved neutron economy.
Another primary object of the present invention is to provide an efficient neutron reflector at the core periphery so as to decrease the amount of fuel enrichment which would otherwise be required to achieve the same length of core life.
Another primary object of the present invention is to provide an efficient neutron reflector at the core periphery so as to increase the flux level at the core periphery and thereby flatten the power distribution across the core.
Other objects although not listed are intended to be within the scope of the present invention. Thus, the above stated objects are not intended to be a complete listing of all the objects of the present invention.